U.S. Pat. No. 2,964,559; No. 3,557,173; and No. 3,673,154 among other patents disclose reclaiming of oxidation metal catalyst cobalt or cobalt and manganese either from the acetic acid mother liquor or a concentrate thereof after separation of such mother liquor and solid iso- or terephthalic acid precipitate from the suspension of such acids in said mother liquor resulting from the liquid phase oxidation of mixed xylenes or m- or p-xylene with air at an elevated temperature above 100.degree. C. in the presence of acetic acid solution of cobalt or cobalt and manganese, generally as their acetates, at an elevated pressure to maintain at least the acetic acid solvent in the liquid phase.
U.S. Pat. No. 2,964,559 teaches that after separating suspended phthalic acids from acetic acid mother liquor and distilling water and acetic acid from said mother liquor leaving a residue, water extraction of the residue reclaims 93% of the cobalt and 94% of the manganese but also extracts 72 mole percent of the phthalic anhydride as the free acid and 80 to 100% of the nickel, iron and chromium present.
U.S. Pat. No. 3,557,173 is concerned with eliminating o-phthalic acid from the cobalt reclaimed from the acetic acid mother liquor. This is done by dehydrating the acetic acid mother liquor (e.g., by addition of acetic anhydride thereto or by distillation of at least 50% of the acetic acid therefrom) whereby anhydrous cobalt acetate precipitates and is recovered by filtration.
U.S. Pat. No. 3,673,154 is concerned with reclamation of cobalt free of iron and chromium. This is done by distilling acetic acid and water from the mother liquor to a pH above 3 (e.g., pH 3.15 to 4.5) which precipitates iron and chromium, removing the Fe and Cr containing precipitate, adding sodium carbonate to precipitate cobalt carbonate and form a soluble form of nickel. Dissolving cobalt carbonate in acetic acid provides the solvent and metal catalyst for the next oxidation of xylene.
Published Japanese Patent Application No. 14,339/71 also concerned with the rejection of iron group contaminants and oxygen-containing aromatic compounds from reclaimed Co or Co and Mn catalyst metals. This is accomplished by distilling acetic acid from the mother liquor after phthalic acid product separation. The distillation residue is extracted either with water or aqueous alkaline carbonate (e.g. Na.sub.2 CO.sub.3) solution. The water extract solution is buffered to a pH of 4.5 to precipitate basic iron acetate. The filtrate after removal of the iron acetate precipitate is treated with sodium carbonate to precipitate cobalt and manganese as carbonates. The extraction with aqueous alkaline carbonate leaves a solid residue which, after recovery from the aqueous solution, is dissolved in an inorganic acid. Buffering the acid solution to pH 4-5 with sodium acetate precipitates iron group metals so that after their removal, Co and Mn can be precipitated as carbonates.
British Patent Specification No. 1,413,829 is concerned with the rejection of iron group contaminant corrosion metals from cobalt and manganese reclaimed as their carbonates from residues comprising concentrates derived by distilling acetic acid and water from the acetic acid mother liquor after recovery of suspended iso- or terephthalic acid. Such residues are extracted with water in an amount of from 3 to 5 weight parts per weight part of residue because such amounts of water dissolve at 80.degree. C. 90 to 98% of the cobalt and manganese content of the residue and provide an extract solution (after separating insolubles) of pH 3.5-5.0 which dissolves relatively little of the iron group metals present. High quality cobalt and manganese carbonates can be precipitated from such solution after its pH is adjusted preferably to pH in the range of 7 to 8.1 by the use of sodium carbonate and/or bicarbonate.
Said British Patent also discloses that use of water in weight amount equal to the weight of the residue dissolves at 80.degree. C. only 72 to 81% of Co and 66-76% of Mn in the residue.
The foregoing techniques for reclaiming Co and/or Mn, while satisfactory when applied to residues obtained from the production of iso- or terephthalic acid, on their face appear either not applicable to or not suitable for the reclamation of cobalt and manganese from residues obtained from the production of o-phthalic acid or its coproduction with trimellitic acid by the respective neat oxidation of liquid o-xylene or liquid mixture of o-xylene and pseudocumene. Such prior reclamations of catalyst metals by change of water content or pH of acetic acid mother liquors do not appear to be applicable to the present problem. The other prior catalyst metal reclamations do not appear to be suitable because they must use rather high weight ratios of water to residue (3 to 5:1) to extract 90% or more of the residue's catalyst metals contents and at such water to residue ratios would appear to dissolve excessive quantities of the oxygen-containing aromatic compounds in the residue.
Recently there have been developed catalytic air oxidation processes whereby either o-phthalic acid is produced from liquid o-xylene or a mixture of o-phthalic acid and trimellitic acid is produced from co-oxidation of liquid o-xylene and pseudocumene neat, that is without the use of extraneous reaction solvents or diluents. For example, the processes disclosed in U.S. patent applications Ser. No. 867,050, filed Jan. 5, 1978 and Ser. No. 874,127, filed Feb. 1, 1978. The fluid reaction effluents from such "solventless" processes can contain from 70 to 92 weight percent o-phthalic acid or mixture of o-phthalic and trimellitic acids. Such fluid effluents can be directly processed to dehydrate the acids to their intramolecular anhydrides and to evaporate said anhydride and materials of lower boiling point in a single step at a pressure from 760 mm Hg down to 40 mm Hg and obtain partially purified anhydride products by the partial condensation at a temperature above the dew point of water but below the boiling points of the anhydrides whereby the condensates collected are partially purified anhydride products. Such substantially simultaneous dehydration and evaporation leaves a liquid residue which contains from 50 to 85%, preferably 65 to 85%, of the anhydride (phthalic anhydride when o-xylene alone was oxidized and trimellitic anhydride when both o-xylene and pseudocumene were simultaneously oxidized) which acts as a flux to provide a mixture which is liquid and relatively free flowing at the 180.degree. C. to 235.degree. C. temperatures at which such residues are formed.
We have discovered that relatively small amounts of water, substantially less than the 16 to 17 to 1 water to residue ratios of U.S. Pat. No. 2,964,559 or the 3:1 to 5:1 water to residue ratios of the British Patent, quite surprisingly will at temperatures of 75 to 80.degree. C. extract more than 90 weight percent of the catalyst metals and less than 25% of the o-phthalic acid from the residues left after the above preparation and recovery of partially purified intramolecular anhydride products and retain the catalyst metals as solutes even at temperatures of 23.degree. C. to 24.degree. C. We also found that, although a substantial amount of the oxygen-containing aromatic impurity compounds were also dissolved by the small amount of water, unexpectedly a substantial proportion of the dissolved impurity compounds could be rejected by diluting the extract solution with additional water without substantial change of operating temperature.